The present invention relates to a control system of an internal combustion engine provided with a thermal type air flow sensor and more particularly to a control system of an internal combustion engine for correcting both surge voltage and warming-up characteristic of the thermal type air flow sensor when the air flow sensor starts so as to correct the intake air flow detected value according to the initial temperatures of both temperature sensing and heating resistors of the thermal type air flow sensor.
It has been a conventional method that disposes a temperature sensing (thermal type) air flow meter in an intake air pipe of an internal combustion engine so as to detect the intake air flow in the engine. The temperature sensing (thermal type) air flow meter has such internal circuits as a temperature sensing resistor and a heating resistor used to detect the intake air flow respectively. In the temperature sensing (thermal type) air flow meter, the heating resistor is disposed in the intake air passage that receives a current and releases its heat at a constant temperature, thereby the temperature drop to be caused by the intake air is prevented with an increase of the supply of the current. The air flow meter calculates the intake air flow from the supplied current value.
However, such a temperature sensing (thermal type) air flow meter generates a detection error in a period between when the meter is powered and when the heating resistor reaches the normal control temperature. This is why the air flow meter detected value must be corrected for a predetermined time after the sensor is powered. And, to solve the conventional problem, there has been disclosed a technique (JP Patent Application No. 7-318118(1995) (JP Patent Publication (Kokai) No. 9-158758 (1997))) that calculates a warming-up correction factor for correcting an intake air flow signal according to an elapsed time after ignition switch ON and the warming-up characteristic of the temperature sensing (thermal type) air flow sensor using means for measuring a time elapsed after the ignition switch ON and means for estimating the warming-up characteristic of the temperature sensing (thermal type) air flow sensor from the water temperature at the starting time of the sensor.
There is also disclosed another technique (JP Patent Publication (Kokai) No. 6-33825(1994)) for correcting the starting time characteristic of such a thermal type air flow sensor according to the results of the measurements of the last power-on time, the last power-off time, and the current power-on time. Concretely, the correcting technique is employed as means for correcting the starting time characteristic of the thermal type air flow sensor described above when the sensor is powered soon after the power-off state so as to prevent the temperature of the heating resistor from falling nearly to the room temperature after the power-off.
There is disclosed proposed still another technique (JP Patent Publication (Kokai) No. 5-288113(1993)) for correcting the starting time characteristic of such a thermal type air flow sensor according to the results of measurements of the last power-off time and the current power-on time, as well as a time on which a predetermined voltage value of the thermal type air flow sensor is exceeded at starting time of the sensor.
Each of the conventional techniques described above, however, includes a problem that the detection accuracy is deteriorated when the equilibrium is lost from among the temperature of the engine water, the temperature of the temperature sensing resistor, and the temperature of the heating resistor of the thermal type air flow sensor. This is because the initial temperature of the thermal type air flow sensor is estimated from the temperature of the engine water at the starting time of the sensor. Concretely, the initial temperatures of the temperature sensing resistor and the heating resistor of the thermal type air flow sensor do not fall so much even when the engine water is low in temperature if the engine stops before it is warmed up, then it starts again soon. In that state, the equilibrium is lost. If the engine starts in such a state, a detection error occurs.
If the initial temperature of the thermal type air flow sensor is estimated from the power-on and power-off times, the last power-off time must always be measured. Thus, the CPU operation must be continued even at the power-off time of the engine, thereby the sensor needs time measuring means for such a measurement. The power consumption therefore comes to increase, since the CPU power cannot be turned off until the engine cools down completely after its power-off. The power wiring system must be checked.
Further, in the case of the conventional technique for correcting the warming-up characteristic of the thermal type air flow sensor according to the time on which the voltage of the air flow sensor at its starting time exceeds a predetermined voltage, the time depend strongly on the supply voltage. This is because falling of the supply voltage causes degradation in the heating performance of the heating resistor and the temperature sensing resistor, thereby the correction amount varies significantly according to the changes of the supply voltage to be caused by how much the battery is charged, the power consumption variation of each accessory member, a sudden fluctuation in the power source at the time of starter switch ON, etc. This has also been a problem.
Under such circumstances, it is an object of the present invention to provide a control system employed for an internal combustion engine provided with a thermal type air flow sensor. The control system is provided with means for correcting the output of the air flow sensor, which is capable of correcting a detected voltage error according to both of the surge voltage and the supply voltage of the sensor when the sensor is started.
In order to solve the above conventional problems, the control system of the present invention employed for an internal combustion engine is basically provided with means for correcting the output of the thermal type air flow sensor. The output correcting means includes surge time measuring means for measuring a surge time in a value detected by the thermal type air flow sensor when the air flow sensor is powered, as well as supply voltage detecting means. The thermal type air flow sensor output correcting means calculates a warming-up characteristic correction amount for the air flow sensor according to the values of the measured surge time and the detected supply voltage. Concretely, the output correcting means estimates the initial temperature of the thermal type air flow sensor element according to the values of the measured surge time and the detected supply voltage, then calculates the warming-up characteristic correcting amount of the thermal type air flow sensor from the estimated initial temperature.
In the control system of the present invention employed for an internal combustion engine and configured as described above, it is possible to correct an output error of the detected voltage properly according to a high voltage output (surge) caused by, for example, a temperature rise of the air flow sensor heating resistor to occur just after the air flow sensor is started. Consequently, the intake air amount of the engine is detected accurately when the engine is started. The starting time variation is thus reduced, thereby the CO and the hydrocarbon included in the exhaust gas is reduced at the starting time of the engine.
The high voltage output (surge) is detected in the temperature rising process of the heating resistor and the high voltage output (surge) time is measured by the surge time measuring means. The surge time of the heating resistor often depends on the supply voltage (ex., battery), so that it is possible to correct the surge time according to the value of the supply voltage so as to correct the output error of the thermal type air flow sensor, thereby calculating the intake air amount of the engine accurately.
Further, in another concrete aspect of the control system of the present invention employed for an internal combustion engine, the thermal type air flow sensor output correcting means includes means for correcting the measured value of the surge time according to the supply voltage during the surge time measurement. The output correcting means calculates a squared average value of the values obtained respectively by subtracting a predetermined value 1 from the supply voltage during a surge time measurement at the starting time of the air flow sensor, then calculates a warming-up characteristic correction amount for the air flow sensor according to the squared average value. Then, the output correcting means estimates a surge time from the squared average value in the normal condition, then calculates the warming-up characteristic correction amount for the thermal type air flow sensor from the estimated surge time. Further, the output correcting means uses a predetermined value 2 as a supply voltage at a resetting time and a total of the measured value and a predetermined value 3 as a surge time to calculate the squared average value of the values obtained respectively by subtracting the predetermined value 1 from the supply voltage during a surge time measurement.
The control system of the present invention employed for an internal combustion engine and configured as described above can correct the surge time measured in the state of the supply voltage during a surge time measurement. Consequently, the surge time is estimated more accurately. It is also possible to correct the surge time measured according to the heating performance calculated during the surge time of the heating resistor of the air flow sensor by subtracting the internal loss of the transistors and resistors in the driving circuit of the air flow sensor from the supply voltage during the surge time measurement by calculating the squared average value of the values obtained respectively by subtracting the predetermined value 1 from the supply voltage during the surge time measurement. Thus, the surge time is estimated more accurately.
Further, according to still another concrete aspect of the present invention, in the control system employed for an internal combustion engine, the predetermined value 3 is a total time of a standard ECU hardware resetting time and a standard software resetting time. The predetermined value 2 is a supply voltage value stored beforehand in a memory system provided in the ECU when the ignition switch is on and the engine isn""t driven. The predetermined value 2 is a value assumed just after the ECU is powered. In addition, the warming-up characteristic correction is not made for the thermal type air flow sensor if no surge voltage is detected when the air flow sensor is started after the ECU is powered.
The control system of the present invention employed for an internal combustion engine and configured as described above estimates a surge time obtained by adding a total time of a standard ECU hardware resetting time and a standard software resetting time to a measured value, so that the surge time is estimated accurately and it is possible to determine the temperature of the heating resistor of the air flow sensor to be high from the beginning if no surge voltage is detected in the air flow sensor actuated after the ECU is powered. The intake air amount of the engine is thus detected without making no warming-up characteristic correction for the air flow sensor.
Further, according to still another aspect of the present invention, the control system employed for an internal combustion engine further includes intake air amount detecting means other than the thermal type air flow sensor, determining means for determining whether or not the value detected by the intake air amount detecting means is equal to or smaller than a predetermined value, and correcting means for correcting a warming-up characteristic correction amount for the air flow sensor. The warming-up characteristic correction amount is corrected by the correcting means only when the intake air amount is determined to be equal to or smaller than the predetermined value. And, the intake air amount detecting means other than the thermal type air flow sensor uses at least one of the throttle opening, the accelerator opening, and the engine speed.
Because the control system of the present invention employed for an internal combustion engine and configured as described above is provided with the intake air amount detecting means other than the thermal type air flow sensor, it is possible to determine whether a detected high voltage of the air flow sensor is caused by a surge or intake air amount, thereby determining whether to request the correcting means to make a correction. In addition, because another detection signal obtained from the throttle opening, the accelerator opening, or the engine speed is used to detect an input to the intake air amount detecting means, no special detecting means is needed.